Full beam metrology for X-ray scatterometry systems

ABSTRACT

Methods and systems for characterizing dimensions and material properties of semiconductor devices by full beam x-ray scatterometry are described herein. A full beam x-ray scatterometry measurement involves illuminating a sample with an X-ray beam and detecting the intensities of the resulting zero diffraction order and higher diffraction orders simultaneously for one or more angles of incidence relative to the sample. The simultaneous measurement of the direct beam and the scattered orders enables high throughput measurements with improved accuracy. The full beam x-ray scatterometry system includes one or more photon counting detectors with high dynamic range and thick, highly absorptive crystal substrates that absorb the direct beam with minimal parasitic backscattering. In other aspects, model based measurements are performed based on the zero diffraction order beam, and measurement performance of the full beam x-ray scatterometry system is estimated and controlled based on properties of the measured zero order beam.

CROSS REFERENCE TO RELATED APPLICATION

The present application for patent claims priority under 35 U.S.C. § 119from U.S. provisional patent application Ser. No. 62/409,758, filed Oct.18, 2016, the subject matter of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The described embodiments relate to metrology systems and methods, andmore particularly to methods and systems for improved measurementaccuracy.

BACKGROUND INFORMATION

Semiconductor devices such as logic and memory devices are typicallyfabricated by a sequence of processing steps applied to a specimen. Thevarious features and multiple structural levels of the semiconductordevices are formed by these processing steps. For example, lithographyamong others is one semiconductor fabrication process that involvesgenerating a pattern on a semiconductor wafer. Additional examples ofsemiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing, etch, deposition, and ion implantation.Multiple semiconductor devices may be fabricated on a singlesemiconductor wafer and then separated into individual semiconductordevices.

Metrology processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield. A number of metrology based techniques including scatterometryand reflectometry implementations and associated analysis algorithms arecommonly used to characterize critical dimensions, film thicknesses,composition and other parameters of nanoscale structures.

Traditionally, scatterometry critical dimension (SCR) measurements areperformed on targets consisting of thin films and/or repeated periodicstructures. During device fabrication, these films and periodicstructures typically represent the actual device geometry and materialstructure or an intermediate design. As devices (e.g., logic and memorydevices) move toward smaller nanometer-scale dimensions,characterization becomes more difficult. Devices incorporating complexthree-dimensional geometry and materials with diverse physicalproperties contribute to characterization difficulty. For example,modern memory structures are often high-aspect ratio, three-dimensionalstructures that make it difficult for optical radiation to penetrate tothe bottom layers. Optical metrology tools utilizing infrared to visiblelight can penetrate many layers of translucent materials, but longerwavelengths that provide good depth of penetration do not providesufficient sensitivity to small anomalies. In addition, the increasingnumber of parameters required to characterize complex structures (e.g.,FinFETs), leads to increasing parameter correlation. As a result, theparameters characterizing the target often cannot be reliably decoupledwith available measurements.

In one example, longer wavelengths (e.g. near infrared) have beenemployed in an attempt to overcome penetration issues for 3D FLASHdevices that utilize polysilicon as one of the alternating materials inthe stack. However, the mirror like structure of 3D FLASH intrinsicallycauses decreasing light intensity as the illumination propagates deeperinto the film stack. This causes sensitivity loss and correlation issuesat depth. In this scenario, SCD is only able to successfully extract areduced set of metrology dimensions with high sensitivity and lowcorrelation.

In another example, opaque, high-k materials are increasingly employedin modern semiconductor structures. Optical radiation is often unable topenetrate layers constructed of these materials. As a result,measurements with thin-film scatterometry tools such as ellipsometers orreflectometers are becoming increasingly challenging.

In response to these challenges, more complex optical metrology toolshave been developed. For example, tools with multiple angles ofillumination, shorter illumination wavelengths, broader ranges ofillumination wavelengths, and more complete information acquisition fromreflected signals (e.g., measuring multiple Mueller matrix elements inaddition to the more conventional reflectivity or ellipsometric signals)have been developed. However, these approaches have not reliablyovercome fundamental challenges associated with measurement of manyadvanced targets (e.g., complex 3D structures, structures smaller than10 nm, structures employing opaque materials) and measurementapplications (e.g., line edge roughness and line width roughnessmeasurements).

Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM)are able to achieve atomic resolution, but they can only probe thesurface of the specimen. In addition, AFM and STM microscopes requirelong scanning times. Scanning electron microscopes (SEM) achieveintermediate resolution levels, but are unable to penetrate structuresto sufficient depth. Thus, high-aspect ratio holes are not characterizedwell. In addition, the required charging of the specimen has an adverseeffect on imaging performance. X-ray reflectometers also suffer frompenetration issues that limit their effectiveness when measuring highaspect ratio structures.

To overcome penetration depth issues, traditional imaging techniquessuch as TEM, SEM etc., are employed with destructive sample preparationtechniques such as focused ion beam (FIB) machining, ion milling,blanket or selective etching, etc. For example, transmission electronmicroscopes (TEM) achieve high resolution levels and are able to probearbitrary depths, but TEM requires destructive sectioning of thespecimen. Several iterations of material removal and measurementgenerally provide the information required to measure the criticalmetrology parameters throughout a three dimensional structure. But,these techniques require sample destruction and lengthy process times.The complexity and time to complete these types of measurementsintroduces large inaccuracies due to drift of etching and metrologysteps. In addition, these techniques require numerous iterations whichintroduce registration errors.

X-Ray scatterometry systems have shown promise to address challengingmeasurement applications. However current implementations block zeroorder light and collect only the scattered orders. This approachintroduces a number of drawbacks. First, the direct beam and scatteredorders are not collected simultaneously. Second, high signal to noiseratio (SNR) beam and system information are lost as the direct beam ismany orders brighter than the collected scattered orders. Finally,signal information embedded in the total scattered light as a functionof the scattering angles is lost.

In spite of these deficiencies, measurements that rely on scatteredorders only provide enough signal information to determine some specimenproperties of interest as long as the incident flux is very stable orwell characterized. In some examples, the incident flux is measuredperiodically, so that flux deviations can be corrected in themeasurement. However, in some measurement applications this approach istoo slow, insufficiently accurate, or both.

In current x-ray scatterometry systems, the direct beam is blocked forseveral reasons. For one, the available detectors are unable to resolvesignals with large dynamic range. In typical semiconductor measurementapplications, scattered signals are typically five to seven orders ofmagnitude less than the direct beam. In addition, the relatively highflux direct beam can trap charge in the detector or saturate the sensorbeyond the damage threshold.

To further improve device performance, the semiconductor industrycontinues to focus on vertical integration, rather than lateral scaling.Thus, accurate measurement of complex, fully three dimensionalstructures is crucial to ensure viability and continued scalingimprovements. Future metrology applications present challenges formetrology due to increasingly small resolution requirements,multi-parameter correlation, increasingly complex geometric structuresincluding high aspect ratio structures, and increasing use of opaquematerials. Thus, methods and systems for improved x-ray scatterometrymeasurements are desired.

SUMMARY

Methods and systems for characterizing dimensions and materialproperties of semiconductor devices by full beam x-ray scatterometry aredescribed herein.

In one aspect, a full beam x-ray scatterometry measurement involvesilluminating a sample with an X-ray beam and detecting the intensitiesof the resulting zero diffraction order and higher diffraction orderssimultaneously for one or more angles of incidence relative to thesample. The simultaneous measurement of the direct beam and thescattered orders enables high throughput measurements with improvedaccuracy.

In another aspect, a full beam x-ray scatterometry system includes oneor more photon counting detectors with high dynamic range (e.g., greaterthan 10⁵) and thick, highly absorptive crystal substrates that absorbthe direct beam without damage and with minimal parasiticbackscattering. In some embodiments, a single photon counting detectordetects the position and number of detected photons. In someembodiments, the x-ray detector resolves one or more x-ray photonenergies.

In a further aspect, the detector is scanned relative to the incomingX-rays to mitigate damage or excessive charging from the incident zeroorder beam.

In another further aspect, overlapping diffraction orders on thedetector are deconvolved based on the measured zero order beam shape.

In another further aspect, the zero order beam profile is extractedduring measurements to mitigate drift during measurement.

In another further aspect, the intensity of higher diffraction orders isestimated relative to the measured zero order beam. In some embodiments,the intensity of each higher diffraction order is estimated relative tothe measured zero order beam by simple division of intensity, orotherwise. In this manner, measurement uncertainty associated with therelatively weak, higher order signals is significantly reduced.

In another aspect, the measurement quality and performance of the fullbeam x-ray scatterometry system is estimated based on properties of themeasured zero order beam. The measured properties of the zero order beaminclude, but are not limited to beam shape, intensity, location,profile, tilt, rotation, asymmetry, or any combination thereof.

In a further aspect, the measurement quality and performance of themetrology system is controlled based on the measured zero order beam. Insome examples, the estimates of measurement quality and performancedescribed hereinbefore are provided as input to a feedback controller.The feedback controller communicates control commands that result inchanges in state of one or more elements of the metrology system thatimproves measurement system quality and performance.

In some examples, metrology based on full beam x-ray scatterometryinvolves determining the dimensions of the sample by the inversesolution of a pre-determined measurement model with the measured data.The measurement model includes a few (on the order of ten) adjustableparameters and is representative of the geometry and optical propertiesof the specimen and the optical properties of the measurement system.The method of inverse solve includes, but is not limited to, model basedregression, tomography, machine learning, or any combination thereof. Inthis manner, target profile parameters are estimated by solving forvalues of a parameterized measurement model that minimize errors betweenthe measured scattered x-ray intensities and modeled results.

In another further aspect, the measured zero order intensity is providedas input to the measurement model during regression.

In another further aspect, the divergence of the measured orders isconsidered in the measurement model associated with a model basedmeasurement of the metrology target.

In another further aspect, a metrology system is configured to generatea structural model (e.g., geometric model, material model, or combinedgeometric and material model) of a measured structure of a specimen,generate a full beam x-ray scatterometry response model that includes atleast one geometric parameter from the structural model, and resolve atleast one specimen parameter value by performing a fitting analysis ofmeasurement data with the response model. In this manner, a comparisonof simulated full beam x-ray scatterometry signals with measured dataenables the determination of geometric as well as material propertiessuch as electron density and elemental identification and composition ofthe sample.

In further aspect, an initial estimate of values of one or moreparameters of interest is determined based on full beam x-rayscatterometry measurements performed at a single orientation of theincident x-ray beam with respect to the measurement target. The initial,estimated values are implemented as the starting values of theparameters of interest for a regression of the measurement model withmeasurement data collected from measurements at multiple orientations.In this manner, a close estimate of a parameter of interest isdetermined with a relatively small amount of computational effort, andby implementing this close estimate as the starting point for aregression over a much larger data set, a refined estimate of theparameter of interest is obtained with less overall computationaleffort.

In a further aspect, full beam x-ray scatterometry measurement data isused to generate an image of a measured structure based on the measuredintensities of the detected diffraction orders. In some embodiments, aresponse function model is generalized to describe the scattering from ageneric electron density mesh. Matching this model to the measuredsignals, while constraining the modelled electron densities in this meshto enforce continuity and sparse edges, provides a three dimensionalimage of the sample.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not limiting in any way. Other aspects,inventive features, and advantages of the devices and/or processesdescribed herein will become apparent in the non-limiting detaileddescription set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrative of a metrology system 100 configured toperform full beam x-ray scatterometry measurements in accordance withthe methods described herein.

FIG. 2 is a diagram illustrative of a metrology system 200 in anotherembodiment configured to perform full beam x-ray scatterometrymeasurements in accordance with the methods described herein.

FIG. 3 depicts an image 171 of scattered orders measured by a full beammetrology system such as metrology system 100.

FIG. 4 depicts an image 172 of scattered orders measured by a full beammetrology system such as metrology system 100.

FIG. 5 depicts a plot 173 of the intensity profile associated with thecross-section, C, of image 172 depicted in FIG. 4.

FIG. 6 depicts the scattering efficiency of a zero order beam as afunction of angle of incidence.

FIG. 7 depicts the scattering efficiency of several higher orders as afunction of angle of incidence.

FIG. 8 is a diagram illustrative of elements of metrology systems 100and 200 contained in vacuum environments separate from specimen 101.

FIG. 9 is a diagram illustrative of a model building and analysis engine150 configured to resolve specimen parameter values based on full beamx-ray scatterometry data in accordance with the methods describedherein.

FIGS. 10A-10C depict an isometric view, a top view, and across-sectional view, respectively, of a typical 3D FLASH memory device190 subject to measurement in the manner described herein.

FIG. 11 depicts x-ray illumination beam 117 incident on wafer 101 at aparticular orientation described by angles ϕ and θ.

FIG. 12 depicts a top view of an array of high aspect ratio holestructures 310.

FIG. 13A depicts a side view of an ideal high aspect ratio holestructure 320.

FIG. 13B depicts a side view of a tilted hole structure 321.

FIG. 13C depicts a side view of a progressively tilted hole structure322, where the degree of tilt progressively increases with depth.

FIG. 14 depicts a flowchart illustrative of an exemplary method 300 ofmeasuring structures based on full beam x-ray scatterometry measurementsas described herein.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

Methods and systems for characterizing dimensions and materialproperties of semiconductor devices by full beam x-ray scatterometry aredescribed herein. Such systems and techniques are employed to measurestructural and material characteristics associated with differentsemiconductor fabrication processes. In some examples, full beam x-rayscatterometry is employed to measure critical dimensions, thicknesses,overlay, and material properties of high aspect ratio semiconductorstructures including, but not limited to, spin transfer torque randomaccess memory (STT-RAM), three dimensional NAND memory (3D-NAND) orvertical NAND memory (V-NAND), dynamic random access memory (DRAM),three dimensional FLASH memory (3D-FLASH), resistive random accessmemory (Re-RAM), and phase change random access memory (PC-RAM).

In one aspect, a full beam x-ray scatterometry measurement involvesilluminating a sample with an X-ray beam and detecting the intensitiesof the resulting zero order and higher diffraction orders simultaneouslyfor one or more angles of incidence relative to the sample. Thesimultaneous measurement of the direct beam and the scattered ordersenables high throughput measurements with improved accuracy. In someembodiments, the 0th order beam is made available by performingmeasurements without a canonical beam block.

The use of high brightness, full beam x-ray scatterometry enables highflux x-ray radiation penetration into opaque areas of the target.Examples of measureable geometric parameters using full beam x-rayscatterometry includes pore size, pore density, line edge roughness,line width roughness, side wall angle, profile, critical dimension,overlay, edge placement error, and pitch. An example of a measureablematerial parameter includes electron density. In some examples, fullbeam x-ray scatterometry enables the measurement of features smallerthan 10 nm as well as advanced semiconductor structures such as STT-RAM,V-NAND, DRAM, PC-RAM and Re-RAM, where measurements of geometricalparameters and material parameters are needed.

Typical x-ray scatterometry systems employ a beam block to block thezero order beam while higher diffraction orders are collected. However,in many semiconductor metrology applications, this prevents successfulmeasurement. For logic devices in the back-end-of-line (BEOL) portion ofthe processing flow, as well as memory (e.g., VNAND and DRAM) in thefront-end-of-line (FEOL) portion of the processing flow, pattern pitchvalues are such that with typical CD-SAXS systems the 1st diffractionorder and the zero order experience a significant amount of spatialoverlap on the detector. If a beam block is used to reject the zeroorder, a portion of the 1st order beam is rejected as well. This causesan error in the measurement of the 1st order beam intensity and an errorin any x-ray scatterometry measurement that relies on the 1st orderbeam. Typically, the 1st order beam has much greater diffractionefficiency than higher order beams. Thus, the 1^(st) order beam iscritical to measurement success. Without the 1st order beam, themeasurement must rely on higher orders only. This significantlyincreases measurement time to achieve adequate signal to noise ratio(SNR) because the diffraction efficiency of orders higher than one ismuch weaker than the 1^(st) diffraction order.

FIG. 1 illustrates an embodiment of a metrology tool 100 for measuringcharacteristics of a specimen in accordance with the exemplary methodspresented herein. As shown in FIG. 1, the system 100 may be used toperform full beam x-ray scatterometry measurements over an inspectionarea 102 of a specimen 101 disposed on a specimen positioning system140. In some embodiments, the inspection area 102 has a spot size ofeighty micrometers or less. In some embodiments, the inspection area 102has a spot size of fifty micrometers or less. In some embodiments, theinspection area 102 has a spot size of forty micrometers or less.

In the depicted embodiment, metrology tool 100 includes an x-rayillumination source 110 configured to generate x-ray radiation suitablefor full beam x-ray scatterometry measurements. In some embodiments, thex-ray illumination system 110 is configured to generate wavelengthsbetween 0.01 nanometers and 1 nanometer. X-ray illumination source 110produces an x-ray beam 117 incident on inspection area 102 of specimen101.

In general, any suitable high-brightness x-ray illumination sourcecapable of generating high brightness x-rays at flux levels sufficientto enable high-throughput, inline metrology may be contemplated tosupply x-ray illumination for full beam x-ray scatterometrymeasurements. In some embodiments, an x-ray source includes a tunablemonochromator that enables the x-ray source to deliver x-ray radiationat different, selectable wavelengths.

In some embodiments, one or more x-ray sources emitting radiation withphoton energy greater than 15 keV are employed to ensure that the x-raysource supplies light at wavelengths that allow sufficient transmissionthrough the entire device as well as the wafer substrate. By way ofnon-limiting example, any of a particle accelerator source, a liquidanode source, a rotating anode source, a stationary, solid anode source,a microfocus source, a microfocus rotating anode source, and an inverseCompton source may be employed as x-ray source 110. In one example, aninverse Compton source available from Lyncean Technologies, Inc., PaloAlto, Calif. (USA) may be contemplated. Inverse Compton sources have anadditional advantage of being able to produce x-rays over a range ofphoton energies, thereby enabling the x-ray source to deliver x-rayradiation at different, selectable wavelengths.

Exemplary x-ray sources include electron beam sources configured tobombard solid or liquid targets to stimulate x-ray radiation. FIG. 2depicts a metrology tool 200 for measuring characteristics of a specimenin accordance with the exemplary methods presented herein. Like numberedelements of metrology tool 100 and 200 are analogous. However, in theembodiment depicted in FIG. 2, x-ray illumination source 110 is a liquidmetal based x-ray illumination system. A jet of liquid metal 119 isproduced from a liquid metal container 111 and collected in a liquidmetal collector 112. A liquid metal circulation system (not shown)returns liquid metal collected by collector 112 to liquid metalcontainer 111. The jet of liquid metal 119 includes one or moreelements. By way of non-limiting example, the jet of liquid metal 119includes any of Aluminum, Gallium, Indium, Tin, Thallium, and Bismuth.In this manner, the jet of liquid metal 119 produces x-ray linescorresponding with its constituent elements. In one embodiment, the jetof liquid metal includes a Gallium and Indium alloy. In someembodiments, the x-ray illumination system 110 is configured to generatewavelengths between 0.01 nanometers and 1 nanometer. An electron beamsource 113 (e.g., electron gun) produces a stream of electrons 118 thatis directed by electron optics 114 to the jet of liquid metal 119.Suitable electron optics 114 includes electromagnets, permanent magnets,or a combination of electromagnets and permanent magnets for focusingthe electron beam and directing the beam at the liquid metal jet. Thecoincidence of the jet of liquid metal 119 and the stream of electrons118 produces an x-ray beam 117 incident on inspection area 102 ofspecimen 101.

Methods and systems for generating high brightness, liquid metal x-rayillumination are described in U.S. Pat. No. 7,929,667, issued on Apr.19, 2011, to KLA-Tencor Corp., the entirety of which is incorporatedherein by reference.

In one embodiment, the incident x-ray beam 117 is at the Indium kα lineof 24.2 keV. The x-ray beam is collimated down to less than onemilliradian divergence using multi-layer x-ray optics for full beamx-ray scatterometry measurements.

In some embodiments, the x-ray scattering measurements described hereinare achieved without using a screen located between the x-ray source andthe specimen under measurement. In these embodiments, the measuredintensities of the full beam over a range of angles of incidence,multiple wavelengths, or a combination of both, provide sufficientinformation to resolve a distribution map (i.e., image) of a desiredmaterial property (e.g., complex refractive index, electron density, orabsorptivity) of the measured structure. However, in some otherexamples, a pinhole or another aperture is located on an otherwiseopaque screen that is located between the x-ray source and the specimenunder measurement to improve collimation of the x-ray beam. Theintensity of the diffraction pattern is measured for several positionsof the aperture. In some other embodiments, a screen with apseudo-random aperture pattern is used, and the diffraction pattern ismeasured for multiple screens. These approaches may also be contemplatedto provide additional information to resolve the three-dimensionaldistribution of the desired material property of the measured structure.

In some embodiments, the profile of the incident x-ray beam iscontrolled two or more apertures, slits, or a combination thereof. In afurther embodiment, the apertures, slits, or both, are configured torotate in coordination with the orientation of the specimen to optimizethe profile of the incident beam for each angle of incidence, azimuthangle, or both.

As depicted in FIG. 1, x-ray optics 115 shape and direct incident x-raybeam 117 to specimen 101. In some examples, x-ray optics 115 include anx-ray monochromator to monochromatize the x-ray beam that is incident onthe specimen 101. In one example, a crystal monochromator such as aLoxley-Tanner-Bowen monochromator is employed to monochromatize the beamof x-ray radiation. In some examples, x-ray optics 115 collimate orfocus the x-ray beam 117 onto inspection area 102 of specimen 101 toless than 1 milliradian divergence using multilayer x-ray optics. Insome embodiments, x-ray optics 115 includes one or more x-raycollimating mirrors, x-ray apertures, x-ray beam stops, refractive x-rayoptics, diffractive optics such as zone plates, specular x-ray opticssuch as grazing incidence ellipsoidal mirrors, polycapillary optics suchas hollow capillary x-ray waveguides, multilayer optics, or systems, orany combination thereof. Further details are described in U.S. PatentPublication No. 2015/0110249, the content of which is incorporatedherein by reference it its entirety.

In general, the focal plane of the illumination optics system isoptimized for each measurement application. In this manner, system 100is configured to located the focal plane at various depths within thespecimen depending on the measurement application.

X-ray detector 116 collects x-ray radiation 125 scattered from specimen101 and generates an output signal 126 indicative of properties ofspecimen 101 that are sensitive to the incident x-ray radiation inaccordance with a full beam x-ray scatterometry measurement modality. Insome embodiments, scattered x-rays 125 are collected by x-ray detector116 while specimen positioning system 140 locates and orients specimen101 to produce angularly resolved scattered x-rays.

In one aspect, a full beam x-ray scatterometry system includes one ormore photon counting detectors with high dynamic range (e.g., greaterthan 10⁵) and thick, highly absorptive crystal substrates that absorbthe direct beam (i.e., zero order beam) without damage and with minimalparasitic backscattering. In some embodiments, a single photon countingdetector detects the position and number of detected photons.

Full beam x-ray scatterometry requires collection of the zero order beamalong with higher diffraction orders. The zero order beam is severalorders of magnitude more intense than the other orders. If the zeroorder beam is not fully absorbed in the X-Ray sensitive section of thedetector, it will scatter and generate parasitic signals. The strengthof these parasitic signals limits the dynamic range of the measurement.For example, if the parasitic signal is 10⁻⁴ of the largest flux signal(i.e., the zero order signal), the signals associated with many higherorders will be contaminated. Thus, it is critical that the detector(e.g., detector 116) exhibit high conversion efficiency of X-rays toelectron hole pairs and high X-ray absorption to increase the effectivedynamic range of the full beam metrology.

Exemplary detector materials suitable for full beam x-ray scatterometryinclude Cadmium Telluride (CdTe), Germanium (Ge) and Gallium Arsenide(GaAs) crystals, and others. In some embodiments, the detector materialis selected to provide high conversion efficiency in a narrow energyband corresponding to the source energy.

In some embodiments, the thickness of the detector material is selectedto achieve the desired absorption of incoming X-rays. In someembodiments, the detector is tilted with respect to the incoming X-raybeams (the various diffraction orders) to increase the path length ofthe X-ray beams through the detector material, and thus, increase thetotal amount of absorption.

In some embodiments, dual threshold detectors are employed to improveSNR.

In a further aspect, the x-ray detector resolves one or more x-rayphoton energies and produces signals for each x-ray energy componentindicative of properties of the specimen. In some embodiments, the x-raydetector 116 includes any of a CCD array, a microchannel plate, aphotodiode array, a microstrip proportional counter, a gas filledproportional counter, a scintillator, or a fluorescent material.

In this manner the X-ray photon interactions within the detector arediscriminated by energy in addition to pixel location and number ofcounts. In some embodiments, the X-ray photon interactions arediscriminated by comparing the energy of the X-ray photon interactionwith a predetermined upper threshold value and a predetermined lowerthreshold value. In one embodiment, this information is communicated tocomputing system 130 via output signals 126 for further processing andstorage.

In a further aspect, the detector is scanned relative to the incomingX-rays to mitigate damage or excessive charging from the incident zeroorder beam. In some embodiments, the detector is continuously scannedwith respect to the incoming X-rays to avoid having the zero order beamdwell on a particular location on the detector surface for an extendedperiod of time. In some other embodiments, the detector is periodicallymoved with respect to the incoming X-rays to avoid having the zero orderbeam dwell on a particular location on the detector surface for anextended period of time. In some embodiments, the scanning or periodicmovements are approximately perpendicular to the incoming X-rays. Insome embodiments, the movements are rotational (e.g., the detector isrotated such that a particular location on the detector surface tracesout a circle in space). In some embodiments, the movements are acombination of translational movements that move the point of incidenceof the zero order beam to various different locations on the detectorsurface.

In a further aspect, a full beam x-ray scatterometry system is employedto determine properties of a specimen (e.g., structural parametervalues) based on multiple measured diffraction orders including the zeroorder scattered light. As depicted in FIG. 1, metrology tool 100includes a computing system 130 employed to acquire signals 126generated by detector 116 and determine properties of the specimen basedat least in part on the acquired signals.

In a full beam x-ray scatterometry measurement, a high aspect ratio,vertically manufactured structure diffracts a collimated X-ray beam intodiffraction orders. Each diffraction order travels in a particular,predictable direction. The angular spacing of the diffraction orders isinversely proportional to the lattice constant of the specimen dividedby the wavelength. The diffraction orders are detected by a detectorarray placed at some distance from the wafer. Each pixel of the detectoroutputs a signal that indicates the number of photons that hit thepixel.

The intensities of diffraction orders are of the form I(m, n, θ, ϕ, λ),where {m,n} are integer indices of diffraction orders, {θ, ϕ} areelevation and azimuth angles of the incident beam (i.e., polarcoordinates of the incident chief ray with respect to a coordinatesystem that is fixed to the wafer), and λ is the wavelength of theincident X-ray.

Several noise sources perturb the illumination light as it exits theillumination and propagates toward the specimen. Exemplary disturbancesinclude electron beam current fluctuation, temperature induced opticdrift, etc. The perturbed incident flux is denoted as F₀(1+n₁).

The target scatters the incident radiation in a manner that depends onthe azimuth and elevation angles of the incident beam. The efficiency oflight scattering into orders (m,n) can be defined as S_(mn)(θ, ϕ). Asthe diffracted light propagates from the specimen to the detector, thebeam passes through other scattering media that affect all orderssimilarly with some variation (1+n₂) and parasitic noise (n₃). In thismanner the total intensity I_(mn) of each order measured in a time, t,can be expressed by equation (1).I _(mn) =S _(mn)(θ,ϕ)(1+n ₂)(1+n ₁)F ₀ t+n ₃  (1)

FIG. 3 depicts an image 171 of scattered orders measured by a full beammetrology system such as metrology system 100. As illustrated in FIG. 3,the bright spot in the center of the image is associated with the zeroorder beam.

The intensity of each order can be extracted in many ways. In someembodiments, the diffraction orders are spatially separated at thedetector. In these embodiments, the diffraction orders are individuallydetected by the detector array, and the outputs of pixels associatedwith the same diffraction order are combined (i.e., added). In thismanner, detected diffraction orders are discriminated by accumulatingphoton counts of pixels associated with each particular diffractionorder. This scenario is more likely to occur when measuring relativelysmall pitch features or when measuring with a beam having a relativelysmall divergence.

In some other embodiments, the diffraction orders spatially overlap atthe detector and the pixel outputs cannot simply be combined todetermine the intensity associated with a particular diffraction order.In these embodiments, a measurement model is employed to deconvolve thediffraction orders to discriminate the measured intensity of eachdetected diffraction order. This scenario is more likely to occur whenmeasuring relatively large pitch features or when measuring with a beamhaving a relatively large divergence.

In a further aspect, overlapped orders are deconvolved based on themeasured zero order beam shape. In some embodiments, this deconvolutionis performed in real time. The beam profile of higher diffracted orders(i.e., orders greater than zero) is modeled based on the profile of thezero order beam. FIG. 4 depicts an image 172 of scattered ordersmeasured by a full beam metrology system such as metrology system 100.FIG. 5 depicts a plot 173 of the intensity profile associated with thecross-section, C, of image 172 depicted in FIG. 4. The relatively highintensity zero order beam provides a very accurate beam profile that isused to model the higher diffraction orders.

In another further aspect, the zero order beam profile is extractedduring measurements to mitigate drift during measurement.

In some embodiments, the zero order beam profile is measured with notarget in the beam target. In some embodiments, the zero order beamprofile is measured with a non-scattering target in the beam path suchthat the zero diffraction order is the only beam measured on thedetector. In some embodiments, the zero order beam profile is measuredwith a known target having known scattering properties.

In another further aspect, the intensity of higher diffraction orders isestimated based on the measured zero order beam. In some embodiments,the intensity of each higher diffraction order is estimated relative tothe measured zero order beam by simple division of intensity, orotherwise. In this manner, measurement uncertainty associated with therelatively weak, higher order signals is significantly reduced.

By estimating the intensity of higher diffraction orders based on thesimultaneously measured zero order beam, scattering signals areseparated from system perturbations during data collection.Perturbations due to misalignment of optical components (e.g., slits,optics, spot shape) and perturbations along the beam path (e.g., n₁ andn₂) are mitigated in real-time. By using all scattered intensities,including the zero order, the dependence of scattered intensities onthickness or material density of the measured specimen is isolated fromflux perturbations before and after the wafer.

The scattering efficiency of the measured specimen relates the extractedscattering intensities to the geometry and materials of the metrologytarget for a set of incidence angles {θ, ϕ}. FIG. 6 depicts thescattering efficiency of the zero order beam, S₀₀, as a function ofangle of incidence, θ. S₀₀ depends on the incidence angle becausetransmission through the target decreases at higher incidence angles dueto increased path length. In addition, S₀₀ depends on the incidenceangle because energy leaves the zero order and enters the higherdiffracting orders when the incidence angle is aligned with thescattering of the target (e.g., normal incidence).

FIG. 7 depicts the scattering efficiency of several higher orders as afunction of angle of incidence, θ. Plotline 174 depicts S₁₁, plotline175 depicts S₁₃. plotline 176 depicts S₂₀, and plotline 177 depicts S₂₂.The scattering intensity for all higher orders typically depends on thescattering depth or density. In general, the scattering efficiency ofthe zero order decreases as scattering depth increases, while thescattering efficiency of every other scattered order increases asscattering depth increases.

Estimating the intensity of higher diffraction orders based on thesimultaneously measured zero order beam also increases the measurementsignal to noise ratio (SNR). This enables more precise measurements ofscattering depth and material density, and thus a more precise estimateof the target profile.

In some examples, metrology based on full beam x-ray scatterometryinvolves determining the dimensions of the sample by the inversesolution of a pre-determined measurement model with the measured data.The measurement model includes a few (on the order of ten) adjustableparameters and is representative of the geometry and optical propertiesof the specimen and the optical properties of the measurement system.The method of inverse solve includes, but is not limited to, model basedregression, tomography, machine learning, or any combination thereof. Inthis manner, target profile parameters are estimated by solving forvalues of a parameterized measurement model that minimize errors betweenthe measured scattered x-ray intensities and modeled results.

In another further aspect, the measured zero order intensity is providedas input to the measurement model during regression. When the zero orderis not measured, the value of total flux must be floated along withother model parameters. This results in a loss of precision.Furthermore, for all scattered orders other than the zero order, thescattered intensity is a function of the scattering contrast alone,i.e., the difference between the indices of refraction of the materials.However, for the zero order, the scattered intensity is a function ofthe absolute values (i.e., undifferenced values) of the indices ofrefraction. This additional information improves measurement precision.

In another further aspect, the divergence of the measured orders isconsidered in the measurement model associated with a model basedmeasurement of the metrology target. In some examples, the resolved beamhas a pixel-dependent scattering efficiency because each pixel is anaverage of slightly different incidence angles {θ, ϕ} due to systemdivergence. The inventors have discovered that measurement time can bedecreased by optimizing divergence for model based measurements based onfull beam x-ray scatterometry. Too little divergence results inincreased measurement time and too much divergence leads to excessivecorrelation and loss of measurement precision.

In another aspect, the measurement quality and performance of the fullbeam x-ray scatterometry system is estimated based on properties of themeasured zero order beam. The measured properties of the zero order beaminclude, but are not limited to beam shape, intensity, location,profile, tilt, rotation, asymmetry, or any combination thereof.

In some examples, the brightness of the illumination source is estimatedbased on a summation of all light detected by the detector. In theabsence of external perturbations, the total measured flux depends ontarget absorption only. In some examples, the measurement is performedwithout a target. In these examples, the total measured flux provides adirect estimate of source brightness. In some other examples, a targethaving known absorption characteristics is employed. In these examples,the source brightness is estimated based on the measured flux correctedby the known target absorption.

In some examples, the transmission efficiency of the system is estimatedbased on a summation of all light detected by the detector. In theseexamples, light emitted by the illumination source is measured as itexits the illumination source, but before interaction with theillumination optics. In addition, the light detected by the detector issummed. The ratio of flux between the light detected at the detector andthe light emitted by the illumination source provides an estimate of thetransmission efficiency of the optical system. In some examples, themeasurement is performed without a target. In some other examples, atarget having known absorption characteristics is employed.

In some examples, the relative alignment of the detector to the beamaxis is estimated based on the location of incidence of the zero orderbeam on the detector.

In some examples, defects or misalignments in the optical subsystem areestimated based on the shape (e.g., asymmetries, roughness, rotations)of the zero order beam measured at the detector. Defects or misalignmentof beam shaping optics, slits, apertures, illumination source, etc., maybe characterized in this manner. In many examples, errors in the slopeof an illumination optic manifest themselves as fine structures of thebeam shape detected at the detector. Small variations in the detectedbeam shape correspond to the position of the beam on the illuminationoptic. In addition, the position of the beam on the slits is ascertainedby monitoring the locations of fine structures due to optic slope errorsrelative to the location of sharp edges due to the slits.

In a further aspect, the measurement quality and performance of themetrology system is controlled based on the measured zero order beam. Insome examples, the estimates of measurement quality and performancedescribed hereinbefore are provided as input to a feedback controller(e.g., computing system 130). The feedback controller communicatescontrol commands that result in changes in state of one or more elementsof the metrology system that improves measurement system quality andperformance.

In some examples, the control commands are provided to the illuminationsource. In response, the electrical state of the illumination source isadjusted to change the scanned spot size and shape, illumination power,spot offsets, incident angles, etc.

In some examples, the control commands are provided to one or morepositioning devices that control the location of one or more opticalelements of the metrology system. In response, the one or morepositioning devices changes a position/orientation of one or moreoptical elements to adjust the incidence angles, focal distance betweenthe illumination source and illumination optics, beam positioning,location of the beam spot on the optic to minimize the effects ofsurface roughness, etc.

In general, the estimates and control of measurement quality andperformance as described herein may be performed with or without atarget present in the beam path.

In a further aspect, full beam x-ray scatterometry measurements areperformed over a range of angles of incidence that provide sufficientresolution and depth of penetration to characterize high aspect ratiostructures through their entire depth.

Measurements of the intensity of diffracted radiation as a function ofx-ray incidence angle relative to the wafer surface normal arecollected. Information contained in the multiple diffraction orders istypically unique between each model parameter under consideration. Thus,x-ray scattering yields estimation results for values of parameters ofinterest with small errors and reduced parameter correlation.

In some embodiments, x-ray detector 116 is maintained in the sameatmospheric environment as specimen 101 (e.g., gas purge environment).However, in some embodiments, the distance between specimen 101 andx-ray detector 116 is lengthy and environmental disturbances (e.g., airturbulence) contribute noise to the detected signals. Hence in someembodiments, one or more of the x-ray detectors is maintained in alocalized, vacuum environment separated from the specimen (e.g.,specimen 101) by a vacuum window.

Similarly, in some embodiments, x-ray illumination source 110,illumination optics 115, or both, are maintained in the same atmosphericenvironment as specimen 101 (e.g., gas purge environment). However, insome embodiments, the optical path length between x-ray illuminationsource 110 and illumination optics 115 and the optical path lengthbetween illumination optics 115 and specimen 101 are long andenvironmental disturbances (e.g., air turbulence) contribute noise tothe illumination beam. Hence in some embodiments, the x-ray illuminationsource, the illumination optics 115, or both, are maintained in alocalized, vacuum environment separated from the specimen (e.g.,specimen 101) by a vacuum window.

FIG. 8 is a diagram illustrative of a vacuum chamber 160 containingx-ray illumination source 110 and illumination optics 115 and a vacuumchamber 160 containing x-ray detector 116 in one embodiment. In apreferred embodiment, vacuum chamber 160 includes a substantial portionof the optical path between x-ray illumination source 110 and specimen101, and vacuum chamber 163 includes a substantial portion of theoptical path between specimen 101 and x-ray detector 116. The openingsof vacuum chamber 160 and vacuum chamber 163 are covered by vacuumwindows 161 and 164, respectively. Vacuum windows 161 and 164 may beconstructed of any suitable material that is substantially transparentto x-ray radiation (e.g., Beryllium). Illumination beam 117 passesthrough vacuum window 161 as it propagates toward specimen 101. Afterinteraction with specimen 101, scattered x-ray radiation 125 passesthrough vacuum window 164, enters vacuum chamber 160 and is incident onx-ray detector 116. A suitable vacuum environment 162 is maintainedwithin vacuum chamber 160 to minimize disturbances to the illuminationbeam 117, and a suitable vacuum environment 165 is maintained withinvacuum chamber 163 to minimize disturbances to scattered x-ray radiation125. A suitable vacuum environment may include any suitable level ofvacuum, any suitable purged environment including an inert gas (e.g.,helium), or any combination thereof. In this manner, as much of the beampath as possible is located in vacuum to maximize flux and minimizeperturbations.

In some embodiments, the entire optical system, including specimen 101,is maintained in vacuum. However, in general, the costs associated withmaintaining specimen 101 in vacuum are high due to the complexitiesassociated with the construction of specimen positioning system 140.

In another further aspect, computing system 130 is configured togenerate a structural model (e.g., geometric model, material model, orcombined geometric and material model) of a measured structure of aspecimen, generate a full beam x-ray scatterometry response model thatincludes at least one geometric parameter from the structural model, andresolve at least one specimen parameter value by performing a fittinganalysis of full beam x-ray scatterometry measurement data with the fullbeam x-ray scatterometry response model. The analysis engine is used tocompare the simulated full beam x-ray scatterometry signals withmeasured data thereby allowing the determination of geometric as well asmaterial properties such as electron density of the sample. In theembodiment depicted in FIG. 1, computing system 130 is configured as amodel building and analysis engine configured to implement modelbuilding and analysis functionality as described herein.

FIG. 9 is a diagram illustrative of an exemplary model building andanalysis engine 150 implemented by computing system 130. As depicted inFIG. 9, model building and analysis engine 150 includes a structuralmodel building module 151 that generates a structural model 152 of ameasured structure of a specimen. In some embodiments, structural model152 also includes material properties of the specimen. The structuralmodel 152 is received as input to full beam x-ray scatterometry responsefunction building module 153. full beam x-ray scatterometry responsefunction building module 153 generates a full beam x-ray scatterometryresponse function model 155 based at least in part on the structuralmodel 152. In some examples, the full beam x-ray scatterometry responsefunction model 155 is based on x-ray form factors,F({right arrow over (q)})=∫ρ({right arrow over (r)})e^(−i{right arrow over (q)}·{right arrow over (r)}) d{right arrow over(r)}  (2)where F is the form factor, q is the scattering vector, and ρ(r) is theelectron density of the specimen in spherical coordinates. The x-rayscattering intensity is then given byI({right arrow over (q)})=F*F.  (3)Full beam x-ray scatterometry response function model 155 is received asinput to fitting analysis module 157. The fitting analysis module 157compares the modeled full beam x-ray scatterometry response with thecorresponding measured data to determine geometric as well as materialproperties of the specimen.

In some examples, the fitting of modeled data to experimental data isachieved by minimizing a chi-squared value. For example, for full beamx-ray scatterometry measurements, a chi-squared value can be defined as

$\begin{matrix}{\chi_{SAXS}^{2} = {\frac{1}{N_{SAXS}}{\sum\limits_{j}^{N_{SAXS}}\frac{\left( {{S_{j}^{{SAXS}\mspace{14mu}{model}}\left( {v_{1},\ldots\mspace{11mu},v_{L}} \right)} - S_{j}^{{SAXS}\mspace{14mu}{experiment}}} \right)^{2}}{\sigma_{{SAXS},j}^{2}}}}} & (4)\end{matrix}$

Where, S_(j) ^(SAXS experiment) is the measured full beam x-rayscatterometry signals 126 in the “channel” j, where the index jdescribes a set of system parameters such as diffraction order, energy,angular coordinate, etc. S_(j) ^(SAXS model)(V₁, . . . , V_(L)) ismodeled full beam x-ray scatterometry signal S_(j) for the “channel” j,evaluated for a set of structure (target) parameters V₁, . . . , V_(L),where these parameters describe geometric (CD, sidewall angle, overlay,etc.) and material (electron density, etc.). σ_(SAXS,j) is theuncertainty associated with the jth channel. N_(SAXS) is the totalnumber of channels in the x-ray metrology. L is the number of parameterscharacterizing the metrology target.

Equation (4) assumes that the uncertainties associated with differentchannels are uncorrelated. In examples where the uncertaintiesassociated with the different channels are correlated, a covariancebetween the uncertainties, can be calculated. In these examples achi-squared value for full beam x-ray scatterometry measurements can beexpressed as

$\begin{matrix}{\chi_{SAXS}^{2} = {\frac{1}{N_{SAXS}}\left( {{{\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {model}}\left( {v_{1},\ldots\mspace{11mu},v_{M}} \right)} - {\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {experiment}}} \right)^{T}{V_{SAXS}^{- 1}\left( {{{\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {model}}\left( {v_{1},\ldots\mspace{11mu},v_{M}} \right)} - {\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {experiment}}} \right)}}} & (5)\end{matrix}$

where, V_(SAXS) is the covariance matrix of the SAXS channeluncertainties, and T denotes the transpose.

In some examples, fitting analysis module 157 resolves at least onespecimen parameter value by performing a fitting analysis on full beamx-ray scatterometry measurement data 126 with the full beam x-rayscatterometry response model 155. In some examples, χ_(SAXS) ² isoptimized.

As described hereinbefore, the fitting of full beam x-ray scatterometrydata is achieved by minimization of chi-squared values. However, ingeneral, the fitting of full beam x-ray scatterometry data may beachieved by other functions.

The fitting of full beam x-ray scatterometry metrology data isadvantageous for any type of full beam x-ray scatterometry technologythat provides sensitivity to geometric and/or material parameters ofinterest. Specimen parameters can be deterministic (e.g., CD, SWA, etc.)or statistical (e.g., rms height of sidewall roughness, roughnesscorrelation length, etc.) as long as proper models describing full beamx-ray scatterometry beam interaction with the specimen are used.

In general, computing system 130 is configured to access modelparameters in real-time, employing Real Time Critical Dimensioning(RTCD), or it may access libraries of pre-computed models fordetermining a value of at least one specimen parameter value associatedwith the specimen 101. In general, some form of CD-engine may be used toevaluate the difference between assigned CD parameters of a specimen andCD parameters associated with the measured specimen. Exemplary methodsand systems for computing specimen parameter values are described inU.S. Pat. No. 7,826,071, issued on Nov. 2, 2010, to KLA-Tencor Corp.,the entirety of which is incorporated herein by reference.

In some examples, model building and analysis engine 150 improves theaccuracy of measured parameters by any combination of feed sidewaysanalysis, feed forward analysis, and parallel analysis. Feed sidewaysanalysis refers to taking multiple data sets on different areas of thesame specimen and passing common parameters determined from the firstdataset onto the second dataset for analysis. Feed forward analysisrefers to taking data sets on different specimens and passing commonparameters forward to subsequent analyses using a stepwise copy exactparameter feed forward approach. Parallel analysis refers to theparallel or concurrent application of a non-linear fitting methodologyto multiple datasets where at least one common parameter is coupledduring the fitting.

Multiple tool and structure analysis refers to a feed forward, feedsideways, or parallel analysis based on regression, a look-up table(i.e., “library” matching), or another fitting procedure of multipledatasets. Exemplary methods and systems for multiple tool and structureanalysis is described in U.S. Pat. No. 7,478,019, issued on Jan. 13,2009, to KLA-Tencor Corp., the entirety of which is incorporated hereinby reference.

In one further aspect, metrology tool 100 includes a computing system(e.g., computing system 130) configured to implement beam controlfunctionality as described herein. In the embodiment depicted in FIG. 1,computing system 130 is configured as a beam controller operable tocontrol any of the illumination properties such as intensity,divergence, spot size, polarization, spectrum, and positioning of theincident illumination beam 117.

As illustrated in FIG. 1, computing system 130 is communicativelycoupled to detector 116. Computing system 130 is configured to receivemeasurement data 126 from detector 116. In one example, measurement data126 includes an indication of the measured response of the specimen(i.e., intensities of the diffraction orders). Based on the distributionof the measured response on the surface of detector 116, the locationand area of incidence of illumination beam 117 on specimen 101 isdetermined by computing system 130. In one example, pattern recognitiontechniques are applied by computing system 130 to determine the locationand area of incidence of illumination beam 117 on specimen 101 based onmeasurement data 126. In some examples, computing system 130communicates command signal 137136 to illumination optics 115 to selectthe desired illumination wavelength and redirect and reshapeillumination beam 117 such that incident illumination beam 117 arrivesat the desired location and angular orientation with respect to specimen101. In some other examples, computing system 130 communicates a commandsignal to wafer positioning system 140 to position and orient specimen101 such that incident illumination beam 117 arrives at the desiredlocation and angular orientation with respect to specimen 101. In someother examples, computing system 130 communicates a command signal 137to x-ray source 110 to select the desired illumination wavelength andredirect and reshape illumination beam 117 such that incidentillumination beam 117 arrives at the desired location and angularorientation with respect to specimen 101.

In some embodiments, it is desirable to perform measurements atdifferent orientations described by rotations about the x and y axesindicated by coordinate system 146 depicted in FIG. 1. This increasesthe precision and accuracy of measured parameters and reducescorrelations among parameters by extending the number and diversity ofdata sets available for analysis to include a variety of large-angle,out of plane orientations. Measuring specimen parameters with a deeper,more diverse data set also reduces correlations among parameters andimproves measurement accuracy. For example, in a normal orientation,full beam x-ray scatterometry is able to resolve the critical dimensionof a feature, but is largely insensitive to sidewall angle and height ofa feature. However, by collecting measurement data over a broad range ofout of plane angular positions, the sidewall angle and height of afeature can be resolved.

As illustrated in FIG. 1, metrology tool 100 includes a specimenpositioning system 140 configured to both align specimen 101 and orientspecimen 101 over a large range of out of plane angular orientationswith respect the scatterometer. In other words, specimen positioningsystem 140 is configured to rotate specimen 101 over a large angularrange about one or more axes of rotation aligned in-plane with thesurface of specimen 101. In some embodiments, specimen positioningsystem 140 is configured to rotate specimen 101 within a range of atleast 90 degrees about one or more axes of rotation aligned in-planewith the surface of specimen 101. In some embodiments, specimenpositioning system is configured to rotate specimen 101 within a rangeof at least 60 degrees about one or more axes of rotation alignedin-plane with the surface of specimen 101. In some other embodiments,specimen positioning system is configured to rotate specimen 101 withina range of at least one degree about one or more axes of rotationaligned in-plane with the surface of specimen 101. In this manner, angleresolved measurements of specimen 101 are collected by metrology system100 over any number of locations on the surface of specimen 101. In oneexample, computing system 130 communicates command signals to motioncontroller 145 of specimen positioning system 140 that indicate thedesired position of specimen 101. In response, motion controller 145generates command signals to the various actuators of specimenpositioning system 140 to achieve the desired positioning of specimen101.

By way of non-limiting example, as illustrated in FIG. 1, specimenpositioning system 140 includes an edge grip chuck 141 to fixedly attachspecimen 101 to specimen positioning system 140. A rotational actuator142 is configured to rotate edge grip chuck 141 and the attachedspecimen 101 with respect to a perimeter frame 143. In the depictedembodiment, rotational actuator 142 is configured to rotate specimen 101about the x-axis of the coordinate system 146 illustrated in FIG. 1. Asdepicted in FIG. 1, a rotation of specimen 101 about the z-axis is an inplane rotation of specimen 101. Rotations about the x-axis and they-axis (not shown) are out of plane rotations of specimen 101 thateffectively tilt the surface of the specimen with respect to themetrology elements of metrology system 100. Although it is notillustrated, a second rotational actuator is configured to rotatespecimen 101 about the y-axis. A linear actuator 144 is configured totranslate perimeter frame 143 in the x-direction. Another linearactuator (not shown) is configured to translate perimeter frame 143 inthe y-direction. In this manner, every location on the surface ofspecimen 101 is available for measurement over a range of out of planeangular positions. For example, in one embodiment, a location ofspecimen 101 is measured over several angular increments within a rangeof −45 degrees to +45 degrees with respect to the normal orientation ofspecimen 101.

In general, specimen positioning system 140 may include any suitablecombination of mechanical elements to achieve the desired linear andangular positioning performance, including, but not limited togoniometer stages, hexapod stages, angular stages, and linear stages.

In further aspect, an initial estimate of values of one or moreparameters of interest is determined based on full beam x-rayscatterometry measurements performed at a single orientation of theincident x-ray beam with respect to the measurement target. The initial,estimated values are implemented as the starting values of theparameters of interest for a regression of the measurement model withmeasurement data collected from full beam x-ray scatterometrymeasurements at multiple orientations. In this manner, a close estimateof a parameter of interest is determined with a relatively small amountof computational effort, and by implementing this close estimate as thestarting point for a regression over a much larger data set, a refinedestimate of the parameter of interest is obtained with less overallcomputational effort.

In a further aspect, full beam x-ray scatterometry measurement data isused to generate an image of a measured structure based on the measuredintensities of the detected diffraction orders. In some embodiments, afull beam x-ray scatterometry response function model is generalized todescribe the scattering from a generic electron density mesh. Matchingthis model to the measured signals, while constraining the modelledelectron densities in this mesh to enforce continuity and sparse edges,provides a three dimensional image of the sample.

Although, geometric, model-based, parametric inversion is preferred forcritical dimension (CD) metrology based on full beam x-ray scatterometrymeasurements, a map of the specimen generated from the same full beamx-ray scatterometry measurement data is useful to identify and correctmodel errors when the measured specimen deviates from the assumptions ofthe geometric model.

In some examples, the image is compared to structural characteristicsestimated by a geometric, model-based parametric inversion of the samescatterometry measurement data. Discrepancies are used to update thegeometric model of the measured structure and improve measurementperformance. The ability to converge on an accurate parametricmeasurement model is particularly important when measuring integratedcircuits to control, monitor, and trouble-shoot their manufacturingprocess.

In some examples, the image is a two dimensional (2-D) map of electrondensity, absorptivity, complex index of refraction, or a combination ofthese material characteristics. In some examples, the image is a threedimensional (3-D) map of electron density, absorptivity, complex indexof refraction, or a combination of these material characteristics. Themap is generated using relatively few physical constraints. In someexamples, one or more parameters of interest, such as critical dimension(CD), sidewall angle (SWA), overlay, edge placement error, pitch walk,etc., are estimated directly from the resulting map. In some otherexamples, the map is useful for debugging the wafer process when thesample geometry or materials deviate outside the range of expectedvalues contemplated by a parametric structural model employed formodel-based CD measurement. In one example, the differences between themap and a rendering of the structure predicted by the parametricstructural model according to its measured parameters are used to updatethe parametric structural model and improve its measurement performance.Further details are described in U.S. Patent Publication No.2015/0300965, the content of which is incorporated herein by referenceit its entirety. Additional details are described in U.S. PatentPublication No. 2015/0117610, the content of which is incorporatedherein by reference it its entirety.

In a further aspect, model building and analysis engine 150 is employedto generate models for combined x-ray and optical measurement analysis.In some examples, optical simulations are based on, e.g., rigorouscoupled-wave analysis (RCWA) where Maxwell's equations are solved tocalculate optical signals such as reflectivities for differentpolarizations, ellipsometric parameters, phase change, etc.

Values of one or more parameters of interest are determined based on acombined fitting analysis of the detected intensities of the x-raydiffraction orders at the plurality of different angles of incidence anddetected optical intensities with a combined, geometricallyparameterized response model. The optical intensities are measured by anoptical metrology tool that may or may not be mechanically integratedwith an x-ray metrology system, such as systems 100 and 200 depicted inFIGS. 1 and 2, respectively. Further details are described in U.S.Patent Publication No. 2014/0019097 and U.S. Patent Publication No.2013/0304424, the contents of each are incorporated herein by referenceit their entirety.

As described herein, full beam x-ray scatterometry measurements areperformed at multiple orientations of the illuminating x-ray beamrelative to the surface normal of the semiconductor wafer. Eachorientation is described by any two angular rotations of wafer 101 withrespect to the x-ray illumination beam, or vice-versa. In one example,the orientation can be described with respect to a coordinate systemfixed to the wafer. FIG. 11 depicts x-ray illumination beam 117 incidenton wafer 101 at a particular orientation described by angles θ and ϕ.Coordinate frame XYZ is fixed the metrology system and coordinate frameX′Y′Z′ is fixed to wafer 101. Z is aligned with an axis normal to thesurface of wafer 101. X and Y are in a plane aligned with the surface ofwafer 101. Similarly, Z′ is aligned with an axis normal to the surfaceof wafer 101, and X′ and Y′ are in a plane aligned with the surface ofwafer 101. As depicted in FIG. 11, x-ray illumination beam 117 lieswithin the X′Z′ plane. Angle, ϕ, describes the orientation of the x-rayillumination beam 117 with respect to the surface normal of the wafer inthe X′Z′ plane. Furthermore, angle, θ, describes the orientation of theX′Z′ plane with respect to the XZ plane. Together, θ and ϕ, uniquelydefine the orientation of the x-ray illumination beam 117 with respectto the surface of wafer 101. In this example, the orientation of thex-ray illumination beam with respect to the surface of wafer 101 isdescribed by a rotation about an axis normal to the surface of wafer 101(i.e., Z axis) and a rotation about an axis aligned with the surface ofwafer 101 (i.e., Y′ axis). In some other examples, the orientation ofthe x-ray illumination beam with respect to the surface of wafer 101 isdescribed by a rotation about a first axis aligned with the surface ofwafer 101 and another axis aligned with the surface of wafer 101 andperpendicular to the first axis as described with reference to FIG. 1.

In some embodiments, the metrology target characterized by full beamx-ray scatterometry measurements as described herein is located within ascribe line of the wafer under measurement. In these embodiments, themetrology target is sized to fit within the width of the scribe line. Insome examples, the scribe line width is less than eighty micrometers. Insome examples, the scribe line is less than fifty micrometers. Ingeneral, the width of the scribe lines employed in semiconductormanufacturing is trending smaller.

In some embodiments, the metrology target characterized full beam x-rayscatterometry measurements as described herein is located within anactive die area of the wafer under measurement and is a part of afunctional integrated circuit (e.g., memory, image sensor, logic device,etc.).

In general, it is preferred that the illumination beam spot size closelymatch the lateral dimensions of the metrology target under measurementto minimize contamination signals from structures surrounding themetrology target under measurement. In some embodiments, the metrologytarget under measurement is less than 70 micrometers in any lateraldimension. In some embodiments, the metrology target under measurementis less than 50 micrometers in any lateral dimension. In someembodiments, the metrology target under measurement is less than 40micrometers in any lateral dimension. In some embodiments, the metrologytarget under measurement is less than 10 micrometers in any lateraldimension. In some embodiments, the metrology target under measurementis characterized by an overall height (or equivalently, depth) of morethan one micrometer. In some embodiments, the metrology target undermeasurement is characterized by an overall height (or equivalently,depth) of more than two micrometers.

In general, a metrology target is characterized by an aspect ratiodefined as a maximum height dimension (i.e., dimension normal to thewafer surface) divided by a maximum lateral extent dimension (i.e.,dimension aligned with the wafer surface) of the metrology target. Insome embodiments, the metrology target under measurement has an aspectratio of at least twenty. In some embodiments, the metrology target hasan aspect ratio of at least forty.

FIGS. 10A-10C depict an isometric view, a top view, and across-sectional view, respectively, of a typical 3D FLASH memory device190 subject to measurement in the manner described herein. The totalheight (or equivalently depth) of memory device 190 ranges from one toseveral micrometers. Memory device 190 is a vertically manufactureddevice. A vertically manufactured device, such as memory device 190,essentially turns a conventional, planar memory device 90 degrees,orienting the bit line and cell string vertically (perpendicular towafer surface). To provide sufficient memory capacity, a large number ofalternating layers of different materials are deposited on the wafer.This requires patterning processes to perform well to depths of severalmicrons for structures with a maximum lateral extent of one hundrednanometers or less. As a result, aspect ratios of 25 to 1 or 50 to 1 arenot uncommon.

FIG. 12 depicts a top view of an array of high aspect ratio holestructures 310. As depicted in FIG. 7, the array of hole structures aremost closely patterned along planes 311, 312, 313, and 314 (which extendinward and outward from the drawing). In some embodiments, it ispreferred to perform measurements of high aspect ratio structures asdescribed herein at orientations of the incident x-ray illumination beamwith respect to the surface of the wafer under measurement that liewithin planes where an array of high aspect ratio structures are mostclosely patterned. In the example depicted in FIG. 12, it is preferredto provide x-ray illumination to the array of hole structures 310 withinplanes 311 and 312, and 313 and 314, where the array of hole structuresare most closely patterned.

FIG. 13A depicts a side view of an ideal high aspect ratio holestructure 320. FIG. 13B depicts a side view of a tilted hole structure321. FIG. 13C depicts a side view of a progressively tilted holestructure 322, where the degree of tilt progressively increases withdepth. In many examples, hole structures 321 and 322 are undesirable. Insome embodiments, hole structures resembling hole structures 321 and 322are characterized by full beam x-ray scatterometry measurements asdescribed herein. In one example, hole structure 321 is characterized bya tilt angle parameter, α. Furthermore, x-ray illumination beam 117 isprovided to hole structure 321 at an angle, ϕ, with respect to thesurface normal, and at the opposite angle, −ϕ, as described, forexample, with reference to FIG. 11. In some embodiments, differences inmeasured T-SAX signals that arise in these two illumination scenariosprovide sufficient signal information to accurately estimate the tiltangle, α.

In another example, hole structure 322 is piecewise characterized by anumber of tilt angle parameter, α₁, α₂, and α₃. Similarly, x-rayillumination beam 117 is provided to hole structure 322 at an angle, ϕ,with respect to the surface normal, and at the opposite angle, −ϕ, asdescribed, for example, with reference to FIG. 11. In some embodiments,differences in measured T-SAX signals that arise in these twoillumination scenarios provide sufficient signal information toaccurately estimate the tilt angles, α₁, α₂, and α₃.

It should be recognized that the various steps described throughout thepresent disclosure may be carried out by a single computer system 130or, alternatively, a multiple computer system 130. Moreover, differentsubsystems of the system 100, such as the specimen positioning system140, may include a computer system suitable for carrying out at least aportion of the steps described herein. Therefore, the aforementioneddescription should not be interpreted as a limitation on the presentinvention but merely an illustration. Further, the one or more computingsystems 130 may be configured to perform any other step(s) of any of themethod embodiments described herein.

In addition, the computer system 130 may be communicatively coupled tothe detector 116 and the illumination optics 115 in any manner known inthe art. For example, the one or more computing systems 130 may becoupled to computing systems associated with the detector 116 and theillumination optics 115, respectively. In another example, any of thedetector 116 and the illumination optics 115 may be controlled directlyby a single computer system coupled to computer system 130.

The computer system 130 may be configured to receive and/or acquire dataor information from the subsystems of the system (e.g., detector 116 andillumination optics 115, and the like) by a transmission medium that mayinclude wireline and/or wireless portions. In this manner, thetransmission medium may serve as a data link between the computer system130 and other subsystems of the system 100.

Computer system 130 of the metrology system 100 may be configured toreceive and/or acquire data or information (e.g., measurement results,modeling inputs, modeling results, etc.) from other systems by atransmission medium that may include wireline and/or wireless portions.In this manner, the transmission medium may serve as a data link betweenthe computer system 130 and other systems (e.g., memory on-boardmetrology system 100, external memory, or external systems). Forexample, the computing system 130 may be configured to receivemeasurement data (e.g., signals 126) from a storage medium (i.e., memory132 or 180) via a data link. For instance, spectral results obtainedusing a spectrometer of any of detector 116 may be stored in a permanentor semi-permanent memory device (e.g., memory 132 or 180). In thisregard, the measurement results may be imported from on-board memory orfrom an external memory system. Moreover, the computer system 130 maysend data to other systems via a transmission medium. For instance,specimen parameter values 170 determined by computer system 130 may bestored in a permanent or semi-permanent memory device (e.g., memory180). In this regard, measurement results may be exported to anothersystem.

Computing system 130 may include, but is not limited to, a personalcomputer system, mainframe computer system, workstation, image computer,parallel processor, or any other device known in the art. In general,the term “computing system” may be broadly defined to encompass anydevice having one or more processors, which execute instructions from amemory medium.

Program instructions 134 implementing methods such as those describedherein may be transmitted over a transmission medium such as a wire,cable, or wireless transmission link. For example, as illustrated inFIG. 1, program instructions stored in memory 132 are transmitted toprocessor 131 over bus 133. Program instructions 134 are stored in acomputer readable medium (e.g., memory 132). Exemplary computer-readablemedia include read-only memory, a random access memory, a magnetic oroptical disk, or a magnetic tape.

In some embodiments, a scatterometry analysis as described herein isimplemented as part of a fabrication process tool. Examples offabrication process tools include, but are not limited to, lithographicexposure tools, film deposition tools, implant tools, and etch tools. Inthis manner, the results of a full beam x-ray scatterometry analysis areused to control a fabrication process. In one example, full beam x-rayscatterometry measurement data collected from one or more targets issent to a fabrication process tool. The full beam x-ray scatterometrymeasurement data is analyzed as described herein and the results used toadjust the operation of the fabrication process tool.

Scatterometry measurements as described herein may be used to determinecharacteristics of a variety of semiconductor structures. Exemplarystructures include, but are not limited to, FinFETs, low-dimensionalstructures such as nanowires or graphene, sub 10 nm structures,lithographic structures, through substrate vias (TSVs), memorystructures such as DRAM, DRAM 4F2, FLASH, MRAM and high aspect ratiomemory structures. Exemplary structural characteristics include, but arenot limited to, geometric parameters such as line edge roughness, linewidth roughness, pore size, pore density, side wall angle, profile,critical dimension, pitch, and material parameters such as electrondensity, composition, grain structure, morphology, stress, strain, andelemental identification.

FIG. 14 illustrates a method 300 suitable for implementation by themetrology systems 100 and 200 of the present invention. In one aspect,it is recognized that data processing blocks of method 300 may becarried out via a pre-programmed algorithm executed by one or moreprocessors of computing system 130. While the following description ispresented in the context of metrology systems 100 and 200, it isrecognized herein that the particular structural aspects of metrologysystems 100 and 200 do not represent limitations and should beinterpreted as illustrative only.

In block 301, a measurement target formed on a wafer surface isilluminated with a focused beam of x-ray radiation at a plurality ofdifferent orientations with respect to the measurement target.

In block 302, an intensity associated with a zero diffraction order andan intensity associated with a higher diffraction order of an amount ofradiation scattered from the measurement target in response to theincident beam of x-ray radiation are simultaneously detected at eachorientation.

In block 303, a value of a parameter of interest associated with a modelof the measurement target is determined based on the detectedintensities of the diffraction orders at the plurality of orientations.

As described herein, the term “critical dimension” includes any criticaldimension of a structure (e.g., bottom critical dimension, middlecritical dimension, top critical dimension, sidewall angle, gratingheight, etc.), a critical dimension between any two or more structures(e.g., distance between two structures), and a displacement between twoor more structures (e.g., overlay displacement between overlayinggrating structures, etc.). Structures may include three dimensionalstructures, patterned structures, overlay structures, etc.

As described herein, the term “critical dimension application” or“critical dimension measurement application” includes any criticaldimension measurement.

As described herein, the term “metrology system” includes any systememployed at least in part to characterize a specimen in any aspect,including critical dimension applications and overlay metrologyapplications. However, such terms of art do not limit the scope of theterm “metrology system” as described herein. In addition, the metrologysystems described herein may be configured for measurement of patternedwafers and/or unpatterned wafers. The metrology system may be configuredas a LED inspection tool, edge inspection tool, backside inspectiontool, macro-inspection tool, or multi-mode inspection tool (involvingdata from one or more platforms simultaneously), and any other metrologyor inspection tool that benefits from the measurement techniquesdescribed herein.

Various embodiments are described herein for a semiconductor processingsystem (e.g., an inspection system or a lithography system) that may beused for processing a specimen. The term “specimen” is used herein torefer to a wafer, a reticle, or any other sample that may be processed(e.g., printed or inspected for defects) by means known in the art.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples include, butare not limited to, monocrystalline silicon, gallium arsenide, andindium phosphide. Such substrates may be commonly found and/or processedin semiconductor fabrication facilities. In some cases, a wafer mayinclude only the substrate (i.e., bare wafer). Alternatively, a wafermay include one or more layers of different materials formed upon asubstrate. One or more layers formed on a wafer may be “patterned” or“unpatterned.” For example, a wafer may include a plurality of dieshaving repeatable pattern features.

A “reticle” may be a reticle at any stage of a reticle fabricationprocess, or a completed reticle that may or may not be released for usein a semiconductor fabrication facility. A reticle, or a “mask,” isgenerally defined as a substantially transparent substrate havingsubstantially opaque regions formed thereon and configured in a pattern.The substrate may include, for example, a glass material such asamorphous SiO₂. A reticle may be disposed above a resist-covered waferduring an exposure step of a lithography process such that the patternon the reticle may be transferred to the resist.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies, each havingrepeatable pattern features. Formation and processing of such layers ofmaterial may ultimately result in completed devices. Many differenttypes of devices may be formed on a wafer, and the term wafer as usedherein is intended to encompass a wafer on which any type of deviceknown in the art is being fabricated.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,XRF disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A metrology system comprising: an x-rayillumination source configured to generate an amount of x-ray radiation;an x-ray illumination optics subsystem configured to illuminate ameasurement target formed on a wafer surface with an amount of x-rayradiation at a plurality of orientations with respect to the measurementtarget; an x-ray detector configured to simultaneously detect anintensity associated with a zero diffraction order and an intensityassociated with a higher diffraction order of an amount of radiationscattered from the measurement target in response to the incident x-rayradiation at each orientation; and a computing system configured to:determine a value of a parameter of interest associated with a model ofthe measurement target based on the detected intensities of thediffraction orders at the plurality of different orientations; anddetermine an indication of measurement quality and performance based ona property of the detected zero diffraction order.
 2. The metrologysystem of claim 1, wherein the zero diffraction order and the higherdiffraction order overlap at the x-ray detector.
 3. The metrology systemof claim 1, wherein the model of the measurement target includes a valueof divergence of the x-ray radiation from the x-ray illumination sourceto the x-ray detector.
 4. The metrology system of claim 1, wherein theindication of measurement quality and performance is any of an alignmentof the x-ray detector to an axis of the incident x-ray radiation, abrightness of the x-ray illumination source, an alignment of the x-rayillumination source, an element of the x-ray illumination opticssubsystem, or both.
 5. The metrology system of claim 1, wherein thecomputing system is further configured to: communicate a command signalto an element of the metrology system to adjust the metrology systembased on the indication of measurement quality and performance.
 6. Themetrology system of claim 1, wherein the computing system is furtherconfigured to: determine a model of the higher diffraction order basedon a measured profile of the zero diffraction order.
 7. The metrologysystem of claim 1, wherein the computing system is further configuredto: estimate an intensity of the higher diffraction order relative tothe measured intensity of the zero diffraction order by dividing themeasured intensity of the higher diffraction order by the measuredintensity of the zero diffraction order.
 8. The metrology system ofclaim 1, wherein a photo-sensitive volume of the x-ray detector includesCadmium Telluride, Germanium, Gallium Arsenide, or any combinationthereof.
 9. The metrology system of claim 1, wherein the measurementtarget includes one or more high aspect ratio structures.
 10. Themetrology system of claim 9, wherein the one or more high aspect ratiostructures is any of a spin transfer torque random access memory(STT-RAM), a three dimensional NAND memory (3D-NAND), a dynamic randomaccess memory (DRAM), a three dimensional FLASH memory (3D-FLASH),resistive random access memory (Re-RAMPC), and a phase change randomaccess memory (PC-RAM).
 11. The metrology system of claim 1, wherein thex-ray illumination source includes any of a liquid metal jet x-rayillumination source, a solid anode x-ray illumination source, and aninverse Compton x-ray illumination source.
 12. The metrology system ofclaim 1, wherein the determining the at least one parameter of interestinvolves a fitting analysis of the detected intensities of thediffraction orders with a geometrically parameterized response model.13. The metrology system of claim 1, wherein the computer subsystem isfurther configured to determine a multi-dimensional image of themeasurement target based on the detected intensities of the diffractionorders at the plurality of different orientations by matching a fullbeam x-ray scatterometry response function model generalized as ageneric electron density mesh to the detected intensities of thediffraction orders at the plurality of different orientations.
 14. Amethod comprising: illuminating a measurement target formed on a wafersurface with an amount of x-ray radiation at a plurality of differentorientations with respect to the measurement target; simultaneouslydetecting an intensity associated with a zero diffraction order and anintensity associated with a higher diffraction order of an amount ofradiation scattered from the measurement target in response to theincident x-ray radiation at each orientation; determining a value of aparameter of interest associated with a model of the measurement targetbased on the detected intensities of the diffraction orders at theplurality of different orientations; and determining an indication ofmeasurement quality and performance based on a property of the detectedzero diffraction order.
 15. The method of claim 14, further comprising:determining an indication of measurement quality and performance basedon a property of the detected zero diffraction order.
 16. The method ofclaim 15, wherein the indication of measurement quality and performanceis any of an alignment of the x-ray detector to an axis of the incidentx-ray radiation, a brightness of the x-ray illumination source, analignment of the x-ray illumination source, an element of the x-rayillumination optics subsystem, or both.
 17. The method of claim 15,further comprising: communicating a command signal to an element of themetrology system to adjust the metrology system based on the indicationof measurement quality and performance.
 18. The method of claim 14,further comprising: determining a model of the higher diffraction orderbased on a measured profile of the zero diffraction order.
 19. Themethod of claim 14, further comprising: dividing the intensity of thehigher diffraction order by the intensity of the zero diffraction order.20. A metrology system comprising: an x-ray illumination sourceconfigured to generate an amount of x-ray radiation; an x-rayillumination optics subsystem configured to illuminate a measurementtarget formed on a wafer surface with the amount of x-ray radiation at aplurality of orientations with respect to the measurement target; anx-ray detector configured to simultaneously detect an intensityassociated with a zero diffraction order and an intensity associatedwith a higher diffraction order of an amount of radiation scattered fromthe measurement target in response to the incident x-ray radiation ateach orientation; and a non-transitory, computer-readable medium,comprising: code for causing a computing system to determine a value ofa parameter of interest associated with a model of the measurementtarget based on the detected intensities of the diffraction orders atthe plurality of orientations, and code for causing the computing systemto determine an indication of measurement quality and performance basedon a property of the detected zero diffraction order.
 21. The metrologysystem of claim 20, wherein the zero diffraction order and the higherdiffraction order overlap at the x-ray detector.